1. Field of the Invention
The present invention relates to a method for crystallizing an amorphous semiconductor thin film, and a method for fabricating a poly-crystalline thin film transistor using the same, and more particularly, to a method for crystallizing an amorphous semiconductor thin film, and a method for fabricating a poly-crystalline thin film transistor using the same, in which a poly-crystalline semiconductor thin film is epitaxially grown using a non-metal seed to thereby realize a crystallization of an amorphous semiconductor thin film having no metal pollution and to thus manufacture a thin film transistor having excellent characteristics.
2. Description of the Related Art
In general, a thin film transistor which is used in a display device such as a liquid crystal display (LCD) or an organic light emitting diode (OLED) is fabricated by processes of depositing amorphous silicon on a transparent substrate such as glass or quartz, forming a gate insulation film and a gate electrode, injecting impurities into a source and a drain, and then annealing the impurities-injected source and drain to thereby activate the injected impurities.
By the way, as a display device using a thin film transistor gradually requires for a fast operating speed and becomes compact in size, a degree of integration of a driving integrated circuit (IC) becomes large and an aperture ratio of a pixel region becomes reduced. Accordingly, an electron mobility on a silicon film should be heightened so that a driving circuit is formed simultaneously together with a pixel thin film transistor (TFT), and an aperture ratio of each pixel should be heightened.
For this purpose, amorphous silicon is crystallized to form poly-crystalline silicon having a large electron mobility. Poly-crystalline silicon is largely divided into high-temperature poly-crystalline silicon obtained by processes of depositing amorphous silicon on a quartz substrate, and then heat-treating the amorphous silicon at a high temperature, and low-temperature poly-crystalline silicon obtained by processes of depositing amorphous silicon on a glass substrate, and then heat-treating the amorphous silicon at a low temperature. Here, a low-temperature poly-crystalline silicon fabrication technology is divided into an eximer laser annealing (ELA) method and a metal induced lateral crystallization (MILC) method which uses crystallization induced metal such as nickle as a catalyzer.
The eximer laser annealing (ELA) method requires for expansive equipment and crystallize silicon by a scanning process, and thus has defects in view of surface roughness and crystallization uniformity. In contrast, the MILC method can utilize conventional heat-treatment equipment and has merits of having a relatively low processing temperature and a relatively short processing time.
However, since the MILC method uses crystallization induced metal, electrical characteristics of a thin film transistor deteriorate due to metal pollution. That is, a leakage current of the thin film transistor increases and an electron mobility thereof decreases. Although various methods have been developed in order to suppress them, fundamental problems have not been solved yet.
A conventional eximer laser annealing (ELA) method of fabricating a poly-crystalline thin film transistor using a conventional method will follow.
FIGS. 1A through 1D are cross-sectional views for explaining a method of fabricating a poly-crystalline thin film transistor using an eximer laser annealing (ELA) method according to the conventional art.
Referring to FIG. 1A, a buffer layer 10 made of an oxide film is formed on an insulation substrate (not shown) such as a glass substrate, and then an amorphous silicon film 11 is formed thereon. Then, laser 12 is irradiated on the amorphous silicon film 11 by a scanning process, to thereby crystallize amorphous silicon film 11.
Thereafter, referring to FIG. 1B, the crystallized silicon film 11 is patterned by a photographic etching process, to thereby form a semiconductor layer 11a. Then, an insulation film and a metal film are sequentially deposited on the substrate, and patterned using the photographic etching process, to thereby form a gate insulation film 13 and a gate electrode 14.
Then, as shown in FIG. 1C, N-type or P-type dopant ions are injected into a source region 11S and a drain region 11D on the substrate using the gate electrode 14 as a mask. Here, a reference designation 11C denotes a channel region.
Thereafter, referring to FIG. 1D, the source region 11S and the drain region 11D are heat-treated by a scanning process in order to electrically activate the dopant ions injected thereinto.
FIGS. 2A through 2D are cross-sectional views for explaining a method of fabricating a thin film transistor using a conventional metal induced lateral crystallization (MILC) method.
Referring to FIG. 2A, a buffer layer 20 made of an oxide film is formed on an insulation substrate (not shown) such as a glass substrate. Then an amorphous silicon film of 500 Å is formed thereon and is patterned by a photographic etching process, to thereby form a semiconductor layer 21. Then, an insulation film and a metal film are sequentially deposited on the substrate, and patterned using the photographic etching process, to thereby form a gate electrode 23 and a gate insulation film 22.
Referring to FIG. 2B, N-type or P-type dopants are ion-injected on the substrate using the gate electrode 23 as a mask, to thereby form a source region 21S and a drain region 21D. As a result, a channel region 21C is defined between the source region 21S and the drain region 21D.
Referring to FIG. 2C, an off-sect structure of approximately 2 μm is formed using a photosensitive agent 24 larger than the pattern of the gate electrode 23. Crystallization induced metal 25 such an nickle (Ni) is deposited by about 50 Å in thickness on the entire surface of the photosensitive agent 24 by a sputtering method.
Thereafter, as shown in FIG. 2D, the photosensitive agent 24 is removed by a lift-off method. Accordingly, the crystallization induced metal 25 is patterned into crystallization induced metal pattern 25a so as to locally remain only in some portions of the source region 21S and the drain region 21D. Thereafter, the substrate is heat-treated using the crystallization induced metal pattern 25a at a temperature of about 580° C. under the hydrogen atmosphere, to thereby crystallize the source region 21S, the drain region 21D, and the channel region 21C, by a metal induced crystallization (MIC) method and the MILC method.
As described above, the conventional crystallization method using a conventional laser heat treatment process requires for expansive equipment and has non-uniform crystallization and inferior surface roughness. As a result, a production cost becomes high and a yield is lowered. Also, metal pollution occurring on a metal film deposited on the amorphous silicon surface for the conventional MILC method exists in the poly-crystalline silicon film, to thus deteriorate an electrical feature of a device (see IEEE Trans. Electron Devices, Vol. 40, No. 5, p. 404, 1993).